JP5446885B2 - Cold rolled steel sheet manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a cold-rolled steel sheet. More specifically, the present invention relates to a method for producing a high-tensile cold-rolled steel sheet that is used after being formed into various shapes by press working or the like, and in particular, a method for producing a high-tensile cold-rolled steel sheet excellent in ductility and stretch flangeability. About.

  Now that the industrial technology field is highly divided, materials used in each technical field are required to have special and high performance. For example, even cold-rolled steel sheets used by press forming are required to have better formability with the diversification of press shapes. In addition, high strength is required, and application of high-tensile cold-rolled steel sheets is being studied. In particular, regarding automotive steel sheets, the demand for thin-walled, high-formability, high-tensile cold-rolled steel sheets has been remarkably increasing in order to reduce the weight of the vehicle body and improve fuel efficiency in consideration of the global environment. In press molding, since the thinner the steel sheet used, the easier it is to crack and wrinkle, a steel sheet with better ductility and stretch flangeability is required. However, these press formability and high strength of the steel sheet are contradictory characteristics, and it is difficult to satisfy these characteristics at the same time.

Until now, as a method for improving the press formability of a high-tensile cold-rolled steel sheet, many techniques relating to micronization of the microstructure have been proposed. For example, Patent Document 1 discloses a method for producing an ultrafine-grained high-strength hot-rolled steel sheet that performs rolling with a total rolling reduction of 80% or more in a temperature range near the Ar ₃ point in a hot rolling process. No. 2 discloses a method for producing ultrafine-grained ferritic steel in which rolling at a rolling reduction of 40% or more is continuously performed in the hot rolling process.

However, methods such as these techniques in which large rolling is performed in the hot rolling process are difficult to apply to industrial production because an excessive load is applied to the rolling mill. In addition, there is no description about a method for finely granulating a cold-rolled steel sheet, and according to the study by the present inventors, a fine-grained hot-rolled steel sheet obtained by large rolling is used as a base material, and cold rolling and When annealing is performed, crystal grains are easily coarsened, and it is difficult to obtain a cold-rolled steel sheet having excellent press formability. In particular, in the production of a dual-phase cold-rolled steel sheet having a metal structure composed of a ferrite phase and a low-temperature transformation generation phase that needs to be annealed in a high temperature range of Ac ₁ point or more, coarsening of crystal grains is remarkable. The advantage of the dual-phase cold-rolled steel sheet that is excellent in ductility cannot be enjoyed.

  Patent Document 3 discloses a method for producing a hot-rolled steel sheet having ultrafine grains, in which a reduction in a dynamic recrystallization region is performed by a reduction pass of 5 stands or more in a hot rolling process. However, it is necessary to extremely reduce the temperature drop during hot rolling, and it is difficult to carry out with normal hot rolling equipment. Moreover, although the example which performed cold rolling and annealing after hot rolling is shown, the balance of tensile strength and hole expansibility is bad, and press formability is inadequate.

  Regarding a method for finely granulating a cold-rolled steel sheet, Patent Document 4 discloses a technique of containing a large amount of Ti or Nb and finely pulverizing ferrite in an annealing process. A duplex steel sheet in which martensite and bainite are dispersed in fine ferrite is obtained, the yield ratio is low, the shape freezing property is improved, and the balance between strength and ductility, and the balance between strength and stretch flangeability However, it is difficult to ensure both ductility and stretch flangeability at the same time. In addition, the addition of Ti and Nb not only increases the production cost, but austenite grains become coarse when annealed at a high temperature, so that the appropriate temperature range for annealing is narrow and it is difficult to ensure production stability.

Patent Document 5, subjected at least once before the process of rapid cooling after heating to cold rolling after Ae ₁ point or more Ae ₃ point or less of the temperature and further maintaining the temperature below ₃ points or more ₁ point Ae Ae quenching A method is disclosed in which a two-phase high-strength steel sheet is finely granulated by annealing, and in Patent Document 6, a hot-rolled steel sheet is subjected to heat treatment at 600 ° C. or higher and Ac _one point or lower, and then cold-rolled and annealed. And a method of making a two-phase high-tensile steel sheet into a fine grain is disclosed. In these methods, since a step of performing pretreatment or heat treatment is added, productivity deterioration and production cost increase are significant.

  In Patent Document 7, immediately after hot rolling, it is rapidly cooled to 720 ° C. or less and held in a temperature range of 600 to 720 ° C. for 2 seconds or more, and the obtained hot-rolled steel sheet is subjected to cold rolling and annealing and has a small particle size distribution. A method for producing a cold-rolled steel sheet having a fine grain structure is disclosed.

JP 58-123823 A JP 59-229413 A Japanese Patent Laid-Open No. 11-152544 JP 2004-250774 A JP 2004-232022 A JP 2005-213603 A International Publication No. 2007/15541

  The technique disclosed in Patent Document 7 described above is a fine grain structure having a small particle size distribution by not subjecting the processing strain accumulated in austenite to the ferrite transformation after the hot rolling is completed, and by transforming the processing strain as a driving force. Is an excellent invention in which excellent ductility and thermal stability are obtained. However, as a result of repeated studies by the present inventors, depending on the chemical composition and production conditions of the steel, after cold rolling and annealing, high strength, good ductility and good stretch flangeability cannot be secured at the same time. Turned out to be.

  The present invention has been made to solve such problems, and more specifically, the problem is that of a high-tensile cold-rolled steel sheet having excellent ductility and stretch flangeability and a tensile strength of 590 MPa or more. It is to provide a manufacturing method.

  The present inventors conducted a detailed investigation on the influence of chemical composition and production conditions on the mechanical properties of high-tensile cold-rolled steel sheets. In the present specification, “%” indicating the content of each element in the chemical composition and the precipitate composition means mass%.

  A series of test steels is mass%, C: less than 0.20%, Si: 2.0% or less, Mn: 1.50% or more and 3.50% or less, P: 0.10% or less, S: 0.010% or less, sol. It had a chemical composition consisting of Al: 0.10% or less, N: 0.010% or less, the balance Fe and impurities.

A slab having such a chemical composition is heated to 1200 ° C., and then hot-rolled to a sheet thickness of 2.0 mm in a temperature range of ₃ or more points of Ar. After hot rolling, it is 720 ° C. or less under various cooling conditions. After cooling to the temperature range, air cooling for 5 to 10 seconds, cooling to various temperatures at a cooling rate of 90 ° C./s or less, setting this as the coiling temperature, and charging it into an electric heating furnace maintained at the same temperature Then, after holding for 30 minutes, the furnace was cooled at a cooling rate of 20 ° C./h to simulate slow cooling after winding. The obtained hot-rolled steel sheet was pickled and cold-rolled to a sheet thickness of 1.0 mm at a rolling rate of 50%. Using the continuous annealing simulator, the obtained cold-rolled steel sheet was heated to various temperatures, subjected to soaking treatment for 95 seconds, and then subjected to annealing for cooling.

  Samples for microstructure observation were collected from the hot-rolled steel sheet and the annealed cold-rolled steel sheet, the longitudinal section parallel to the rolling direction was polished, and then scanning electrons equipped with an optical microscope and an Auger electron spectrometer (AES) Using a microscope (SEM), the metal structure was observed from the surface of the steel sheet at a 1/4 depth position of the plate thickness. In addition, a tensile test piece was taken from the annealed cold-rolled steel sheet in parallel with the rolling direction and subjected to a tensile test. Furthermore, the stretch flangeability of the cold-rolled steel sheet subjected to the annealing was evaluated by a hole expansion test. In the hole expansion test, a punched hole with a diameter of 10 mm is formed with a clearance of 12.5%, the punched hole is expanded with a conical punch with a tip angle of 60 °, and the expansion rate of the hole when a crack that penetrates the plate thickness occurs is measured. did.

As a result of these preliminary tests, the following findings (A) to (F) were obtained.
(A) A hot-rolled steel sheet manufactured through a so-called immediate quenching process in which water quenching is performed immediately after hot rolling, specifically, quenching to a temperature range of 720 ° C. or less within 0.4 seconds from completion of hot rolling. When the hot-rolled steel sheet manufactured by cold-rolling is annealed by cold rolling, the ductility and stretch flangeability of the cold-rolled steel sheet improve as the annealing temperature rises, but if the annealing temperature is too high, the austenite grains become coarse, The ductility and stretch flangeability of cold-rolled steel sheets may deteriorate rapidly.

  (B) When the iron carbide is precipitated at a predetermined number density on the ferrite grain boundary in the winding step immediately after the rapid cooling, the coarsening of the austenite grains is suppressed when annealing is performed at a high temperature after cold rolling. The reason for this is not clear, but (a) iron carbide functions as a nucleation site in the transformation from ferrite to austenite during annealing. Therefore, the greater the amount of iron carbide precipitated, the higher the nucleation frequency, and the austenite It is presumed that fine graining (b) undissolved iron carbide is caused by fine graining of austenite in order to suppress grain growth of austenite.

(C) The higher the coiling temperature, the higher the number density of iron carbides precipitated on the ferrite grain boundaries, and the stronger the effect of preventing austenite grain coarsening.
(D) As the Si content in the steel increases, the number density of iron carbides precipitated on the ferrite grain boundaries increases and the effect of preventing the austenite grains from coarsening becomes stronger.

(E) While suppressing the coarsening of austenite grains (Ac ₃ points -40 ° C) and soaking in the temperature range above, cooling the finely dispersed ferrite as the second phase with the fine low-temperature transformation phase as the main phase To obtain a metallographic structure.

(F) A cold-rolled steel sheet having such a metal structure exhibits good ductility and good stretch flangeability while having high strength.
From the above results, a steel containing a certain amount or more of Si is hot-rolled and then immediately cooled, iron carbide is precipitated at a predetermined number density at the ferrite grain boundaries, and cold-rolled (Ac ₃ point-40 To produce a cold-rolled steel sheet that has a structure in which the main phase is a low-temperature transformation generation phase and a ferrite phase is contained in the second phase, and is excellent in ductility and stretch flangeability. Can do.

The present invention completed based on the above knowledge is as follows.
(1) Cold rolling comprising a metallographic structure including the following steps (A) to (C), wherein the main phase is a low-temperature transformation generation phase and the second phase includes ferrite having an average crystal grain size of less than 5 μm Steel plate manufacturing method:
(A) By mass%, C: more than 0.020% and less than 0.20%, Si: more than 0.10% and 2.0% or less, Mn: 1.50% or more and 3.50% or less, P: 0.00. 10% or less, S: 0.010% or less, sol. Al: 0.10% or less and N: 0.010% or less, the slab having a Ru chemical composition Na balance being Fe and impurities, hot rolling is subjected to finish hot rolling the rolling at a temperature range of not lower than the Ar ₃ point The steel sheet is cooled, and the hot rolled steel sheet is cooled to a temperature range of 720 ° C. or less within 0.4 seconds after the completion of the rolling, and the average number density of iron carbide existing on the ferrite grain boundaries is 5.0 ×. Hot rolling step of 10 ⁻² pieces / μm ² or more;
(B) a cold rolling process in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is subjected to a soaking process in a temperature range of (Ac ₃ points-40 ° C) or higher. Annealing process.

  (2) The chemical composition is one or more selected from the group consisting of Ti: less than 0.040%, Nb: less than 0.030%, and V: 0.50% or less in terms of mass%. The method for producing a cold-rolled steel sheet according to (1) above, characterized in that

  (3) The chemical composition is one or more selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.010% or less in terms of mass%. The method for producing a cold-rolled steel sheet according to (1) or (2) above, characterized in that

  (4) The chemical composition is further selected from the group consisting of Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less in terms of mass%. The method for producing a cold-rolled steel sheet according to any one of the above (1) to (3), wherein the cold rolled steel sheet contains one or more kinds.

  According to the present invention, a high-tensile cold-rolled steel sheet having sufficient ductility and stretch flangeability that can be applied to processing such as press forming can be obtained. The present invention greatly contributes to the development of industries, such as contributing to the solution of global environmental problems through weight reduction of automobile bodies.

  The metallographic structure and chemical composition of the high-tensile cold-rolled steel sheet according to the present invention and the rolling and annealing conditions in the production method capable of producing the steel sheet efficiently, stably and economically will be described in detail below.

1. Metal structure The high-tensile cold-rolled steel sheet of the present embodiment has a composite structure in which the main phase is a fine low-temperature transformation generation phase and the second phase contains fine ferrite. This is because it is suitable for increasing the strength without impairing stretch flangeability. Here, the low temperature transformation generation phase refers to a phase and structure produced by low temperature transformation such as martensite and bainite. Other than these, acicular ferrite, bainitic ferrite and tempered martensite are exemplified. This low temperature transformation product phase may contain two or more phases and structures, such as martensite and bainite. The main phase means a phase or structure having the maximum volume fraction, and the second phase means a phase and structure other than the main phase. When the low temperature transformation product phase includes two or more phases and structures, the sum of the volume fractions of these phases is defined as the volume fraction of the low temperature transformation product phase.

  In order to improve stretch flangeability, the volume ratio of the low-temperature transformation generation phase is preferably 60.0% or more. More preferably, it is 70.0% or more, and particularly preferably 80.0% or more. On the other hand, if the volume fraction of the low temperature transformation product phase is excessive, the ductility deteriorates. Therefore, the volume fraction of the low temperature transformation product phase is preferably less than 95.0%. More preferably, it is less than 90.0%.

  In order to improve ductility, the average crystal grain size of ferrite is preferably less than 5.0 μm. More preferably, it is less than 4.0 micrometers, Most preferably, it is less than 3.0 micrometers. From the same point of view, the ferrite volume fraction is preferably more than 3.0%. More preferably, it is more than 8.0%, particularly preferably more than 14.0%.

  In order to increase the tensile strength, the low-temperature transformation generation phase preferably contains martensite. In this case, the volume ratio of martensite in the entire structure is preferably 3.0% or more, more preferably 6.0% or more, and particularly preferably 10.0% or more. On the other hand, when the volume ratio of martensite becomes excessive, stretch flangeability deteriorates. For this reason, the volume ratio of martensite in the entire structure is preferably less than 50.0%, more preferably less than 30.0%, and particularly preferably less than 20.0%. Moreover, even if the volume fraction of bainite becomes excessive, it becomes difficult to ensure high tensile strength, and it is necessary to contain a large amount of Mn, Ti, Nb, etc., resulting in an increase in material cost. Therefore, the volume fraction of bainite in the entire structure is preferably less than 85.0%, more preferably less than 80.0%, and particularly preferably less than 75.0%.

  The second phase may contain retained austenite in addition to ferrite. When retained austenite is included, ductility is improved. However, when the volume ratio of retained austenite becomes excessive, stretch flangeability deteriorates. For this reason, the volume ratio of retained austenite is preferably less than 15.0%, more preferably less than 11.0%, and particularly preferably less than 9.0%. In order to more reliably obtain the effect of improving ductility due to retained austenite, the volume ratio of retained austenite is preferably more than 2.0%, more preferably more than 3.0%, and more than 5.0%. It is particularly preferred.

  In the present embodiment, in the case of a cold-rolled steel sheet, the position of a quarter depth of the sheet thickness from the surface of the steel sheet, and in the case of a plated steel sheet, the steel sheet that is the base material from the boundary between the steel sheet that is the base material and the plating layer. The above-described metal structure is defined at a position of a quarter depth of the plate thickness.

As a mechanical property that can be realized based on the above-described characteristics on the metal structure, the steel plate of the present embodiment preferably has a tensile strength (TS) of 590 MPa or more in order to ensure shock absorption. 780 MPa or more, more preferably 980 MPa or more. Moreover, in order to ensure ductility, it is preferable that TS is less than 1180 MPa. From the viewpoint of press formability, when the total elongation of the steel sheet is El and the hole expansion ratio measured according to the Japan Iron and Steel Federation Standard JFST1001 is λ, the value of TS ² × E1 × λ is 7.5 ×. It is preferably 10 ⁸ MPa ² % ² or more, more preferably 8.0 × 10 ⁸ MPa ² % ² or more, and particularly preferably 8.5 × 10 ⁸ MPa ² % ² or more. From the viewpoint of shape freezeability, the yield ratio is preferably less than 80%, more preferably less than 75%, and particularly preferably less than 70%.

2. Chemical composition of steel C: more than 0.020% and less than 0.20% When the C content is 0.020% or less, it is difficult to obtain the above metal structure. Therefore, the C content is more than 0.020%. Preferably it is more than 0.060%, more preferably more than 0.080%, particularly preferably more than 0.10%. On the other hand, when the C content is 0.20% or more, not only the ductility of the steel sheet is impaired, but also the weldability deteriorates. Therefore, the C content is less than 0.20%. Preferably it is less than 0.17%, More preferably, it is less than 0.15%, Most preferably, it is less than 0.13%.

Si: more than 0.10% and 2.0% or less Si has an action of suppressing austenite grain growth during annealing by increasing the number density of iron carbide existing on the ferrite grain boundaries in the hot-rolled steel sheet, Through this action, the ductility and stretch flangeability of the cold-rolled steel sheet are enhanced. When the Si content is 0.10% or less, it is difficult to obtain the effect by the above-described action. Therefore, the Si content is more than 0.10%. Preferably it is more than 0.25%, more preferably more than 0.50%, particularly preferably more than 0.60%. On the other hand, if the Si content exceeds 2.0%, the surface properties of the steel sheet deteriorate. Furthermore, chemical conversion property and plating property are remarkably deteriorated. Therefore, the Si content is 2.0% or less. Preferably it is 1.50% or less, More preferably, it is 1.25% or less, Most preferably, it is less than 1.00%.

Mn: 1.50% or more and 3.50% or less Mn has an effect of improving the hardenability of steel and is an effective element for obtaining the above metal structure. If the Mn content is less than 1.50%, it is difficult to obtain the above metal structure. Therefore, the Mn content is 1.50% or more. Preferably it is more than 2.10%, more preferably more than 2.20%, particularly preferably more than 2.30%. On the other hand, if the Mn content exceeds 3.50%, the volume fraction of ferrite is too small and ductility deteriorates, and the bendability is impaired due to segregation of Mn, and further, the material cost increases. Therefore, the Mn content is 3.50% or less. Preferably it is less than 3.00%, more preferably less than 2.70%, particularly preferably less than 2.50%.

P: 0.10% or less P is an element contained in steel as an impurity, and segregates at grain boundaries to embrittle the steel. For this reason, the smaller the P content, the better. Therefore, the P content is 0.10% or less. Preferably it is less than 0.020%, more preferably less than 0.015%.

S: 0.010% or less S is an element contained in steel as an impurity, and forms sulfide inclusions to deteriorate stretch flangeability. For this reason, the smaller the S content, the better. Therefore, the S content is 0.010% or less. Preferably it is 0.005% or less, More preferably, it is less than 0.003%, Most preferably, it is 0.001% or less.

sol. Al: 0.10% or less Al has a function of deoxidizing molten steel. In the present invention, since Si having a deoxidizing action is contained in the same manner as Al, Al is not necessarily contained. When it is contained for the purpose of deoxidation, sol. Even if the content of Al exceeds 0.10%, the effect is saturated and uneconomical. The Al content is 0.10% or less. Preferably it is 0.05% or less, More preferably, it is 0.02% or less. In order to obtain the effect of deoxidation by Al more reliably, sol. The Al content is preferably 0.005% or more.

N: 0.010% or less N is an element contained in steel as an impurity, and deteriorates ductility. For this reason, the smaller the N content, the better. Therefore, the N content is 0.010% or less. Preferably it is 0.006% or less, More preferably, it is 0.005% or less.

The steel plate according to the present embodiment may contain the elements listed below as optional elements.
One or more selected from the group consisting of Ti: less than 0.040%, Nb: less than 0.030% and V: 0.50% or less Ti, Nb and V precipitate as carbides or nitrides. , Has the effect of suppressing the austenite coarsening during annealing and improving ductility and stretch flangeability. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. In addition, the recrystallization temperature increases, the metal structure of the cold-rolled steel sheet becomes non-uniform, and the stretch flangeability is also impaired. Furthermore, the precipitation amount of carbide or nitride increases, the yield ratio increases, and the shape freezing property also deteriorates. Therefore, the Ti content is less than 0.040%, the Nb content is less than 0.030%, and the V content is 0.50% or less. The Ti content is preferably less than 0.025%, more preferably less than 0.020%, the Nb content is preferably less than 0.020%, particularly preferably 0.015% or less, and the V content is Preferably it is 0.30% or less. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Ti: 0.005% or more, Nb: 0.005% or more, and V: 0.010% or more. When Ti is contained, the Ti content is more preferably 0.010% or more, and when Nb is contained, the Nb content is more preferably 0.010% or more.

One or more selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.010% or less Cr, Mo, and B improve the hardenability of steel. It is an element effective in obtaining the above metal structure. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. Therefore, the Cr content is 1.0% or less, the Mo content is 0.50% or less, and the B content is 0.010% or less. The Cr content is preferably 0.50% or less, the Mo content is preferably 0.20% or less, and the B content is preferably 0.0030% or less. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Cr: 0.20% or more, Mo: 0.05% or more, and B: 0.0010% or more.

Ca, Mg and REM selected from the group consisting of Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less By adjusting the shape of the inclusions, Bi has the effect of improving stretch flangeability by refining the solidified structure. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. Therefore, the Ca content is 0.010% or less, the Mg content is 0.010% or less, the REM content is 0.050% or less, and the Bi content is 0.050% or less. Preferably, the Ca content is 0.0020% or less, the Mg content is 0.0020% or less, the REM content is 0.0020% or less, and the Bi content is 0.010% or less. In order to obtain the above action more reliably, it is preferable to satisfy any of Ca: 0.0005% or more, Mg: 0.0005% or more, REM: 0.0005% or more, and Bi: 0.0010% or more. . Note that REM means a rare earth element and is a generic name for a total of 17 elements of Sc, Y and lanthanoid, and the REM content is the total content of these elements.

3. Manufacturing conditions The steel having the above-mentioned chemical composition is melted by a known means and then made into a steel ingot by a continuous casting method, or a method of rolling into pieces after making it into an ingot by any casting method, etc. It is made into a billet. In the continuous casting process, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold. The steel ingot or steel slab may be reheated once it has been cooled and subjected to hot rolling. The steel ingot in the high temperature state after continuous casting or the steel slab in the high temperature state after partial rolling is used as it is. Alternatively, it may be kept hot or subjected to auxiliary heating for hot rolling. In the present specification, such steel ingots and steel slabs are collectively referred to as “slabs” as materials for hot rolling. The temperature of the slab to be subjected to hot rolling is preferably less than 1250 ° C. and more preferably 1200 ° C. or less in order to prevent coarsening of austenite. The lower limit of the temperature of the slab to be subjected to hot rolling is not particularly limited as long as it is a temperature at which hot rolling can be completed at Ar ₃ points or more as described later.

Hot rolling is completed in a temperature range of Ar ₃ or more in order to refine the structure of the hot-rolled steel sheet by transforming austenite after completion of rolling. When the temperature at the completion of rolling is low, the texture of the hot-rolled steel sheet develops, and the stretch flangeability after cold rolling and annealing deteriorates. For this reason, it is preferable that the hot rolling is completed at a temperature of (Ar ₃ points + 20 ° C.) or higher. It is more preferable to complete at a temperature of (Ar ₃ points + 30 ° C.) or higher. Moreover, when the temperature of hot rolling is too low, the rolling load increases and rolling becomes difficult. Therefore, it is preferable to complete hot rolling at 780 ° C. or higher, and more preferably at 800 ° C. or higher.

  In addition, when hot rolling consists of rough rolling and finish rolling, in order to complete finish rolling at the said temperature, you may heat a rough rolling material between rough rolling and finish rolling. At this time, it is desirable to suppress the temperature fluctuation over the entire length of the rough rolled material at the start of finish rolling to 140 ° C. or lower by heating so that the rear end of the rough rolled material is higher than the front end. Thereby, the uniformity of the product characteristic in a coil improves.

  The heating method of the rough rolled material may be performed using a known means. For example, a solenoid induction heating device is provided between the rough rolling mill and the finish rolling mill, and the heating temperature rise is controlled based on the temperature distribution in the longitudinal direction of the rough rolled material on the upstream side of the induction heating device. May be.

  The amount of hot rolling reduction is preferably as high as possible in order to increase the amount of processing strain introduced into austenite and promote the refinement of the structure of the hot-rolled steel sheet. However, if the reduction amount becomes too high, the rolling load increases and rolling becomes difficult. Therefore, it is preferable that the reduction amount of the final one pass of hot rolling is more than 15% and less than 50% in terms of sheet thickness reduction rate, and more preferably more than 20% and less than 40%.

  After hot rolling, it is rapidly cooled to a temperature range of 720 ° C. or less within 0.4 seconds after completion of rolling. This suppresses the release of processing strain introduced into austenite by rolling, transforms austenite using processing strain as a driving force, refines the structure of hot-rolled steel sheet, ductility and stretch flangeability after cold rolling and annealing It is for improving. Preferably, it is rapid cooling to a temperature range of 720 ° C. or less within 0.3 seconds after completion of rolling, and more preferably, it is rapidly cooled to a temperature range of 720 ° C. or less within 0.2 seconds after completion of rolling. . In addition, since release of processing strain is suppressed as the average cooling rate during rapid cooling increases, the average cooling rate during rapid cooling is preferably set to 400 ° C./s or more, thereby further increasing the structure of the hot-rolled steel sheet. It can be miniaturized. The average cooling rate during the rapid cooling is more preferably 500 ° C./s or more, and particularly preferably 700 ° C./s or more. In addition, it is not necessary to prescribe | regulate especially the time from the completion of rolling to the start of rapid cooling, and the cooling rate in the meantime.

  The equipment for rapid cooling is not particularly defined, but industrially, it is preferable to use a water spray device with a high water density, and a water spray header is disposed between the rolling plate conveyance rollers, and sufficient from above and below the rolling plate. A method of injecting high-pressure water having a water density is exemplified.

  After the rapid cooling stop, the steel sheet is wound into a coil shape and iron carbide is precipitated on the ferrite grain boundaries. In the present invention, the ductility and stretch flangeability of the cold-rolled steel sheet are particularly improved by increasing the amount of iron carbide precipitated on the ferrite grain boundaries.

Here, the iron carbide means a compound mainly composed of Fe and C, and Fe ₃ C, Fe ₃ (C, B), and Fe ₂₃ (C, B) ₆ are exemplified. In order to efficiently suppress the coarsening of austenite, Fe ₃ C is preferable. Moreover, steel components, such as Mn and Cr, may be dissolved in these iron carbides.

In the present invention, the amount of iron carbide existing on the ferrite grain boundary of the hot-rolled steel sheet is defined by an average number density (unit: pieces / μm ² ), and the average number density of the iron carbide is measured as follows. . That is, a specimen is taken from an arbitrary position of a steel plate, a longitudinal section parallel to the rolling direction is polished, and a metal structure is observed using an optical microscope or SEM at a position of 1/4 depth of the plate thickness from the steel plate surface. Then, the composition of the precipitate is analyzed using AES, and the precipitate containing Fe and C as the constituent elements is iron carbide, and the number density of the iron carbide existing on the grain boundary is measured. In this case, the measurement is performed excluding iron carbide existing on the boundary between ferrite and a phase other than ferrite and the structure.

The average number density of the iron carbide existing on the grain boundary of the hot-rolled steel sheet according to the present invention is 5.0 × 10 ⁻² pieces / μm ² or more. This is for suppressing the austenite coarsening in the annealing process and improving the ductility and stretch flangeability of the cold-rolled steel sheet. The average number density of iron carbide is more preferably 7.0 × 10 ⁻² pieces / μm ² or more, and particularly preferably 9.0 × 10 ⁻² pieces / μm ² or more.

  The conditions from the rapid cooling stop to the winding are not particularly defined, but after the rapid cooling stop, it is preferable to hold at a temperature range of 720 to 600 ° C. for 1 second or more. Thereby, the production | generation of a fine ferrite is accelerated | stimulated. On the other hand, if the holding time becomes too long, the productivity is impaired. Therefore, the upper limit of the holding time in the temperature range of 720 to 600 ° C. is preferably within 10 seconds. After holding in the temperature range of 720 to 600 ° C., it is preferable to cool to the coiling temperature at a cooling rate of 20 ° C./s or more in order to prevent the generated ferrite from becoming coarse.

  The coiling temperature is preferably more than 400 ° C., more preferably more than 500 ° C., and more preferably more than 550 ° C. in order to sufficiently precipitate iron carbide on the grain boundaries and improve ductility and stretch flangeability. This is particularly preferable.

  The hot-rolled steel sheet produced as described above has a metal structure mainly composed of fine ferrite and containing iron carbide. In order to further suppress the coarsening of austenite during annealing and further improve the ductility and stretch flangeability of the cold-rolled steel sheet, the average crystal grain size of ferrite is preferably 3.0 μm or less, and 2.0 μm or less. It is more preferable. Further, the volume ratio of ferrite is preferably 85% or more, and more preferably 90% or more.

  The hot-rolled steel sheet is descaled by pickling or the like and then cold-rolled according to a conventional method. In cold rolling, in order to promote recrystallization to make the metal structure of the cold-rolled steel sheet uniform and improve stretch flangeability, the cold pressure ratio is preferably set to 40% or more. If the cold pressure ratio is too high, the rolling load increases and rolling becomes difficult, so the upper limit of the cold pressure ratio is preferably less than 70%, and more preferably less than 60%.

The steel sheet after cold rolling is annealed after being subjected to a treatment such as degreasing according to a known method, if necessary. The soaking temperature in annealing is set to (Ac ₃ points−40 ° C.) or higher. This is to obtain a metal structure in which the main phase is a low-temperature transformation generation phase and the second phase contains ferrite. In order to increase the volume ratio of the low-temperature transformation generation phase and improve stretch flangeability, the soaking temperature is preferably (Ac ₃ points−20 ° C.), more preferably Ac ₃ points. However, if the soaking temperature is too high, austenite becomes excessively coarse and ductility and stretch flangeability deteriorate. Therefore, the soaking temperature is preferably less than (Ac ₃ points + 100 ° C.), and more preferably less than (Ac ₃ points + 50 ° C.).

  As for the heating rate to the soaking temperature in annealing, the temperature range of 700 ° C. or higher is preferably less than 10 ° C./s. If the heating rate to reach the soaking temperature is too high, the metal structure of the cold-rolled steel sheet becomes non-uniform, and there is a possibility that the stretch flangeability is deteriorated.

  In the cooling process after soaking in annealing, cooling may be performed at a cooling rate of less than 5 ° C./s to a temperature in the temperature range of 750 to 650 ° C. in order to promote the formation of fine polygonal ferrite. Moreover, in order to obtain a low temperature transformation production | generation phase, it is preferable to cool the temperature range of 650-450 degreeC with the cooling rate of 15 to 200 degreeC / s. A more preferable cooling rate is more than 30 ° C./s and less than 150 ° C./s, and a particularly preferable cooling rate is more than 50 ° C./s and less than 130 ° C./s. Moreover, in order to adjust the metal structure of the cold rolled steel sheet, it may be held for 60 seconds or more in a temperature range of 450 to 200 ° C. In order to increase the tensile strength, the holding temperature is preferably 400 ° C. or lower. On the other hand, in order to reduce the hardness difference between the low-temperature transformation generation phase and the ferrite and improve stretch flangeability, the holding temperature is preferably 300 ° C. or higher, and more preferably 350 ° C. or higher.

  When manufacturing a plated steel sheet, the cold-rolled steel sheet manufactured by the above-described method may be electroplated or hot-plated according to a conventional method. The plating method, the chemical composition of the plating film, and the alloying treatment after plating are performed. It is not limited to the presence or absence. Examples of electroplating include electrogalvanizing and electro-Zn-Ni alloy plating. Examples of the hot dip plating include hot dip galvanizing, alloyed hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating and the like. .

  In addition, when manufacturing a hot-dip galvanized steel sheet, in the cooling process after soaking in annealing, the steel sheet is cooled at a cooling rate of 4 ° C./s or higher to a temperature in the temperature range of 600 to 460 ° C. It is preferable to perform hot dip galvanization after holding for 10 seconds or more. Thereby, it becomes easy to obtain a low temperature transformation production phase. Moreover, in order to improve the corrosion resistance after coating, it is preferable to reheat after hot dip galvanizing and alloying treatment.

  The cold-rolled steel sheet and the plated steel sheet thus obtained may be subjected to temper rolling according to a conventional method. However, when the elongation rate of temper rolling is high, ductility is deteriorated. Therefore, the elongation of temper rolling is preferably 1.0% or less. A more preferable elongation is 0.5% or less.

The present invention will be described more specifically with reference to examples.
Steel having the chemical composition shown in Table 1 was melted and cast using a laboratory vacuum melting furnace. These steel ingots were made into steel pieces having a thickness of 30 mm by hot forging. The steel slab was heated to 1200 ° C. using an electric heating furnace and held for 60 minutes, and then hot rolled under the conditions shown in Table 2.

Specifically, using an experimental hot rolling mill, 6-pass rolling was performed in a temperature range of ₃ or more points of Ar, and finished to a thickness of 2 to 3 mm. The reduction rate of the final pass was 22 to 25% in terms of plate thickness reduction rate. After hot rolling, it is cooled to 650-720 ° C. under various cooling conditions using a water spray, allowed to cool for 5-10 seconds, and then cooled to various temperatures at a cooling rate of 60 ° C./s. The temperature is set to the coiling temperature, charged in an electric heating furnace maintained at the same temperature and held for 30 minutes, and then cooled to room temperature at a cooling rate of 20 ° C./h to simulate slow cooling after winding. As a result, a hot-rolled steel sheet was obtained.

  The obtained hot-rolled steel sheet was pickled to form a cold-rolled base material, and cold-rolled at a reduction rate of 50 to 60% to obtain a cold-rolled steel sheet having a thickness of 1.0 to 1.2 mm. Using the continuous annealing simulator, the obtained cold rolled steel sheet was heated to 550 ° C. at a heating rate of 10 ° C./s, and then heated to various temperatures shown in Table 2 at a heating rate of 2 ° C./s. Apply soaking for 95 seconds, then cool to various cooling stop temperatures shown in Table 2 at an average cooling rate from 700 ° C. of 60 ° C./s, hold at that temperature for 330 seconds, then cool to room temperature To be annealed.

Sample specimens for structure observation were taken from the obtained hot-rolled steel sheet, and after polishing a longitudinal section parallel to the rolling direction, the position of the depth of the sheet was 1/4 depth from the steel sheet surface using an optical microscope and SEM / AES. The metal structure in was observed. The average number density of the iron carbide was determined by observing a 10 ⁴ μm ² visual field at a magnification of 1000 times and measuring the number of iron carbides present on the ferrite grain boundaries. Further, the volume fraction of ferrite and the average crystal grain size of ferrite were measured by image processing.

  In addition, a specimen for SEM observation was collected from the annealed cold-rolled steel sheet, and after polishing the longitudinal section parallel to the rolling direction, the metal structure at the 1/4 depth position of the plate thickness was observed from the steel sheet surface. Then, the low temperature transformation generation phase, the volume fraction of ferrite, and the average crystal grain size of ferrite were measured by image processing. Moreover, the test piece for X-ray diffraction was extract | collected and the amount of retained austenite in the 1/4 depth position of plate | board thickness from the steel plate surface was measured by X-ray diffraction.

  Yield stress (YS), tensile strength (TS), and total elongation (El) are obtained by taking a JIS No. 5 tensile specimen along the rolling direction from the annealed cold-rolled steel sheet and conducting a tensile test. It was.

  Stretch flangeability was evaluated by measuring the hole expansion rate (λ) by the following method. A 100 mm square plate is collected from the annealed cold rolled steel sheet, a punched hole with a diameter of 10 mm is formed with a clearance of 12.5%, and the punched hole is expanded from the sag side with a conical punch with a tip angle of 60 °. The expansion rate of the hole when a crack penetrating the plate thickness was measured, and this was defined as the hole expansion rate.

  Table 3 shows the metal structure observation results and performance evaluation results of the cold-rolled steel sheet.

As for the test results (trial numbers 1, 4, 7-9, 11-15) for the steel plates within the range defined by the present invention, the value of TS ² × E1 × λ is 9.0 × 10 ⁸ MPa ^2. % ² or more, showing good ductility and stretch flangeability. Note that the metal structure of each cold-rolled steel sheet was mainly composed of a low-temperature transformation generation phase, and the low-temperature transformation generation phase contained bainite and martensite.

The test results (trial numbers ² , 3, 5, 6, 10) for steel plates whose steel composition or manufacturing method deviates from the range specified by the present invention are all TS ² × E1 × λ values of 5.3. × 10 ⁸ MPa ² % ² or less, and the ductility and stretch flangeability were inferior.

  Specifically, in the test using Steel A (Trial No. 2) and the test using Steel B (Trial No. 5), the time from the completion of hot rolling to the rapid cooling stop is too long. The average number density of the iron carbide existing on the ferrite grain boundary is low, the ferrite of the cold-rolled steel sheet becomes coarse, and the ductility and stretch flangeability are poor. In the test using Steel A (Trial No. 3), since the soaking temperature during annealing was too low, a metal structure having a low-temperature transformation generation phase as a main phase was not obtained, and the ferrite of the cold-rolled steel sheet was coarse. Yes, ductility and stretch flangeability are poor. In the test using Steel B (Trial No. 6), since the coiling temperature is too low, there is no iron carbide on the ferrite grain boundary of the hot-rolled steel sheet, the ferrite of the cold-rolled steel sheet becomes coarse, ductility and elongation. Flangeability is poor. In the test using steel F (trial number 10), since the Si content in the steel is small, the average number density of iron carbide existing on the ferrite grain boundary of the hot-rolled steel sheet is low, and the ferrite of the cold-rolled steel sheet is low. It is coarse and has poor ductility and stretch flangeability.

Claims (4)

Production of a cold-rolled steel sheet having a metal structure containing a ferrite whose main phase is a low-temperature transformation generation phase and whose average crystal grain size is less than 5 μm , which has the following steps (A) to (C) Method:
(A) By mass%, C: more than 0.020% and less than 0.20%, Si: more than 0.10% and 2.0% or less, Mn: 1.50% or more and 3.50% or less, P: 0.00. 10% or less, S: 0.010% or less, sol. Al: 0.10% or less and N: 0.010% or less, the slab having a Ru chemical composition Na balance being Fe and impurities, hot rolling is subjected to finish hot rolling the rolling at a temperature range of not lower than the Ar ₃ point The steel sheet is cooled, and the hot rolled steel sheet is cooled to a temperature range of 720 ° C. or less within 0.4 seconds after the completion of the rolling, and the average number density of iron carbide existing on the ferrite grain boundaries is 5.0 ×. Hot rolling step of 10 ⁻² pieces / μm ² or more;
(B) a cold rolling process in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is subjected to a soaking process in a temperature range of (Ac ₃ points-40 ° C) or higher. Annealing process.
  The chemical composition further contains one or more selected from the group consisting of Ti: less than 0.040%, Nb: less than 0.030%, and V: 0.50% or less by mass%. The method for producing a cold-rolled steel sheet according to claim 1, wherein   The chemical composition further contains, in mass%, one or more selected from the group consisting of Cr: 1.0% or less, Mo: 0.50% or less, and B: 0.010% or less. The method for producing a cold-rolled steel sheet according to claim 1 or 2, wherein the method is a thing.   The chemical composition is further selected from the group consisting of Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less in terms of mass%. The method for producing a cold-rolled steel sheet according to any one of claims 1 to 3, characterized in that it contains seeds or two or more kinds.